The heart is a muscular organ that pumps blood out to the body. The heart of a normal person beats at regular time intervals, while that of a person manifesting symptoms of arrhythmia exhibits an excessively low heart rate or an excessively high heart rate even when no particular external causes are present. Arrhythmias with too slow heart beats (bradycardia) are called “bradyarrhythmias,” and those with too rapid heart beats (tachycardia) are called “tachyarrhythmias.”
Possible causes of arrhythmia include the presence of accessory pathways in the system of cardiac excitation propagation. Accessory pathway, also known as reentry, is an extra conduction path of electrical signals, running separately from a heart's own stimulus conduction system. The problem is that an accessory pathway could divert incoming electrical signals to a wrong direction. If the signal direction is reversed, it makes a local loop of stimulus propagation, which keeps the heart receiving frequent contraction signals and thus causes tachyarrhythmia.
As a medical treatment for arrhythmias resulting from an accessory pathway, catheter ablation is used to cauterize a spot of tissue having abnormal excitation that causes tachycardia. During a catheter ablation procedure, an electrode catheter is inserted into the heart, and a radio-frequency (RF) current is passed between the inserted electrode and a return electrode attached to the body surface to generate heat at the tachycardia-causing spot. The cauterization closes off the harmful accessory pathway, thereby reducing or eliminating occurrence of arrhythmia.
What is particularly important in catheter ablation is how to determine the exact location of an accessory pathway. One method for this is to use a computer to simulate propagation of cardiac excitation.
Simulation of cardiac excitation conduction is a kind of numerical analysis for computationally reproducing functions of a heart, which mimics electrical activities (propagation of excitation) in myocardium with progress of time. Specifically, a heart model of an arrhythmia patient is subjected to this simulation to analyze how the electrical stimulus signal travels in his or her heart's muscle when it is experiencing arrhythmia.
For example, one proposed medical instrument permits analysis of electrical activities in a heart, on the basis of measurement data obtained from a test subject (patient) as to the electrical potential or magnetic field or both. The outcome of this analysis is then visualized for effective use in a subsequent catheter ablation treatment for the patient's arrhythmia. Visualization of electrophysiological data may be achieved by using, for example, an existing technique that provides spatial representation of information on a graphic image of a predetermined surface area of an organ.
As another applicable tool, an electrophysiological mapping system may be used to determine the target location of an ablation surgery. This electrophysiological mapping system enables the user to understand spatial relationships between mapping data and anatomical features of a heart in preparation for catheter-based RF ablation. See, for example, the following documents:    Japanese Laid-open Patent Publication No. 11-128191    Japanese National Publication of International Patent Application No. 2012-508079    Japanese Laid-open Patent Publication No. 2001-070269
The above-noted conventional techniques and systems are, however, not helpful enough for medical practitioners to understand how the excitation signals propagate along an accessory pathway in a patient's heart. That is, it is hard to provide an easy-to-understand view of electrical signal waves that cause cardiac excitation because of their complicated moving directions in a heart under the condition of arrhythmia. While it is possible to generate a picture visualizing electrical activities in the entire heart, the viewer would still be unable to find out which part of the picture contains accessory pathways and is thus likely to overlook them.